Stainless steel pickling



Patented Nov. 15, 1949 STAINLESS STEEL PICKLIN G Frederick 0. Abbott, Harrington, N. J., assignor to The Pennsylvania Salt Manufacturing Company, Philadelphia, Pa., a corporation of Pennsylvania No Drawing. Application August 2, 1946, Serial No. 687,973

Claims. (Cl. 252-446) This invention relates to a new composition and process for the pickling of stainless steel.

Acid baths containing fluorides have been proposed heretofore for the pickling of stainless steel, e. g., for the removal of the oxide scale formed upon the processing of stainless steel articles in processes employing heat. Thus, aqueous solutions of ferric sulphate and hydrofluoric acid have been proposed for the pickling of stainless steel and aqueous solutions of nitric and hydrofluoric acids have been proposed as brightening baths in the pickling of stainless steel. However, compositions and processes heretofore suggested for the pickling of stainless steel have generally necessitated the handling of hydrofluoric acid by the steel treating plants, which is hazardous and expensive, and the procedures employed have usually required, for the bulk of the stainless steel processed commercially, the use of two successive pickling baths in order to produce a surface of desirable characteristics. Thus, of the two HF baths above mentioned the ferric sulphate-hydrofluoric mixture is employed industrially as a primary bath and is followed in the usual commercial practice by a secondary brightening bath consisting of the HNO3-HF mixture. Only in the case of a small proportion of the commercial stainless steel can the latter bath be used as the only pickling bath, and it has serious disadvantages, involving the handling, by steel fabricators, of such hazardous, corrosive and expensive materials as nitric and hydrofluoric acids.

I have now discovered a new stainless steel pickling composition which is surprisingly capable of accomplishing the complete pickling and brightening of stainless steel objects employing only the one bath, and making use of materials more readily handled by steel fabricators. The composition of my invention is a mixture of a. soluble chlorate and a soluble fluoride salt in proportion such that, on dissolving in an appropriate aqueous bath, hereinafter defined, the resulting solution when employed as a pickling and brightening bath for stainless steels, substantially completely obviates the objections to prior art processes. My composition is preferably a solid mixture of chlorate and fluoride salts, which upon solution in a dilute aqueous acid such as sulphuric or hydrochloric acid, or in the case of some species of my composition merely upon I solution in water, forms an acid bath containing chlorate and fluoride ions wh ch, as stated above, is remarkabl advantageous for the complete pickling and brightening of stainless steel.

My process for the pickling of stainless steel involves treatment of the stainless steel objects having oxide scale with an aqueous solution containing suitable concentrations of hydrogen ion, chlorate ion and fluoride ion furnished by a fluoride salt, namely at least about 0.01 percent by weight of hydrogen ion, at least about 0.1 percent by weight of chlorate ion, and at least about 0.5 percent by weight of such fluoride ion. Since both fluoride ion and chlorate ion are furnished by salts in my bath, it is a characteristic of my bath that its content of cations other than hydrogen ion (e. g. metal cations) is always greater than its content of chlorate ion, on the basis of ionic equiv alents, and important advantages appear to follow from this.

In determining suitable concentration ranges for the chlorate ion, an upper limit of about 6 percent by weight concentration is suggested on economic grounds, i. e., more than 6 percent by weight of the ion may be employed, but does not seem to add to the efficacy of the bath, and in some instances such high concentrations may cause excessive attack on the metal. With respect to the minimum concentration of chlorate ion, at least 0.1 percent by weight is important at the start of the operation, i. e., it is important to add at least this amount in the preparation of a new pickling bath. However, I have found, that after pickling has proceeded for an appreciable time the concentration may decrease to a lower value, and may even become so low that the chlorate ion is substantially completely eliminated without appreciably deleteriously alfecting the action of the bath for a limited period of time. However, in the absence of chlorate ion the efiiciency of the bath may become somewhat lower, and therefore I have found it more advantageous to prevent the concentration from decreasing below the stated minimum value. With respect to the upper limit of fluoride ion, a value of about 6 percent by weight is also suggested for economic reasons, coupled with the extent of solubility of the fluoride compound in question. It is clear, therefore, that if the solubility permits, a larger concentration of the fluoride compond may be used, but for the usual applications herein contemplated is unnecessary. With respect to the concentration of hydrogen ion, while it is possible to go higher than about 0.5 percent by weight, I have found that a material increase in the concentration above this value does not appreciably increase the efficiency of the bath in pickling and brightening operations. I have also found that the concentration of hydrogen ion should be maintained at least 3 about 0.01 percent by weight in order to achieve the objects of my invention.

By employing the process and composition of my invention, efiective pickling of stainless steel ling of stainless steel. Otherwise, the concentration of hydrogen ion may be furnished by any of the readily available inexpensive commercial acids, e. g. sulphuric acid, hydrochloric acid, or nitric acid.

to produce a bright finish may be accomplished 5 much more economically than with the pickling The following examples are illustrative of the bath heretofore proposed. Moreover, as above composition and process of my invention: stated, the composition and process of my in- Example 1 vention have the important commercial advantage that a single bath may accomplish both 4 par s y wei ht of olid cry line so ium the primary and secondary pickling of stainless chlorate were well mixed with 4 parts by weight steel. The composition and process of my invenof solid finely divided sodium bifluoride. 8 parts tion have the further advantage that all types of by weight of the resulting mixture were disstainless steel may be pickled by this method and solved in 112 parts by weight of water containing by this bath, whereas in prior art practice it has 14 parts by weight of sulfuric acid. been found necessary to employe different types Stainless steel objects made from steel conof baths for different types of stainless steel. taining:

Object Carbon, Chromium, Nickel, Other Elements, Number Per Cent Per Cent Per Cent Per Cent oiostoaoo 17-19 8-10 Manganese 2.00 maximum.

.08 maximum.. 18-20 8-10 Do. .10 maximum. 16-18 10-14 Manganese and molybdenum 2-3. .12 maximum-- 14-18 Other important advantages in the composition were employed in this test. These objects conand process of my invention, such as ease in tained oxide scale consisting mainly of the oxides handling and reduction of hazard, have been dis- 0 of iron, chromium, and nickel, produced by the cussed above. 3 processing of the steel including the operations In a preferred composition prepared in accordof hot rolling and annealing. ance with my invention, a soluble chlorate such The above described stainless steel objects were as sodium or potassium chlorate or other alkali immersed in the solution of sodium chlorate and metal chlorate in finely divided solid form is sodium bifluoride made as above described. By blended with a soluble fluoride, also in finely this treatment the stainless steel objects were divided solid form. The solid fluoride may adcompletely freed of oxide scale and were given vantageously be sodium or potassium fluoride or a bright finish, requiring no further pickling other alkali metal fluoride, or may be a fluoride treatment, within a period of 2 to 15 minutes. salt of acid reaction such as sodium or potassium 40 Exam 1 2 bifiuoride or other alkali metal bifluoride. Other p e fluorides soluble in acid, e. g. ferric fluoride, may 4 parts by weight of solid crystalline sodium be employed, or in place of the simple fluorides chlorate were mixed well with 4 parts by weight complex fluorides soluble under the conditions of solid finely divided sodium bifluoride and 15 employed, e. g., cryolite, a sodium aluminum i'luparts of sodium acid sulfate. 23 parts by weight oride, may be employed. As above made clear, of the resulting mixture were dissolved in 91 parts in a composition suitable for making up a bath by weight of water containing 6 parts by weight initially, the chlorate and fluoride are employed of 93% sulfuric acid. in such proportions that when dissolved in water Stainless steel objects similar to those used in or dilute acid the solution will contain from Example 1 were immersed in this bath. Within about .02 to 12 parts by weight of chlorate ion 2 to 15 minutes the stainless steel objects were per part by weight of fluoride ion; a preferred completely freed of oxide scale and exhibited a composition will furnish, when dissolved in water bright surface requiring no further pickling. or dilute acid, from about A; to about 6 parts Exam Z 3 by Weight chlorate ion per part by weight fluoride p 8 ion, for example, a mixture of about 20 percent An aqueous pickling bath comprising sulfuric to about 80 percent by weight sodium chlorate, acid y e Sodium fl o de 5% y the balance consisting primarily of a sodium flu- Weight, Sodium chlorate 4% by Weight, and waoride such as sodium bifiuoride, in such a comt r 83% by Weight Was p p d by first mixing position. The resulting pickling bath is also a pulverized sodium fluoride and sodium chlorate, new composition coming within the scope of my and dissolving the mixture in acid containing invention. I have further found that for replenwater. ishing a used fluoride-chlorate pickling bath, a Stainless steel objects similar to those used composition furnishing from about 12 to 40 parts in Example 1 were immersed in this bath. Withby weight of chlorate ion per part fluoride ion is in 2 to 15 minutes the stainless steel objects were quite advantageous. 05 completely freed of oxide scale and exhibited a In employing the above described composition bright surface requiri n fur h r pi kl n for the pickling of stainless steel, the above de- Example 4 scribed mixture of solids may be dissolved in water or in a dilute aqueous solution of sulphuric An aqueous pi klin bath omprisin Sulfuric or hydrochloric acid. When the solid composifi d by weight, Sodium chlorate y tion contains an acid fluoride such as sodium biweight, sodium bifluoride 2% by weight, ferric fluoride, or some other solid added to furnish chloride hexahydrate 3.6% by weight, and water acidity, e. g. sodium acid sulfate, solutions of the 84.4% by weight was prepared by dissolving apcompositions in water alone may contain a sufiipropriate amounts of the various ingredients in cient concentration of hydrogen ion for the pickwater.

Stainless steel objects similar to those used in Example 1 were immersed in this bath. Within 2 to 15 minutes the stainless steel objects were completely freed of oxide scale and exhibited a bright surface requiring no further pickling.

Example 5 The same procedure and materials were used as in Example 4, except 2.5% by weight ferric fluoride was substituted for 3.6% by weight ferric chloride hexahydrate, the balance being water. Substantially identical results were obtained.

The term stainless steel is employed in the specification and claims to denote a ferrous alloy containing from about 8 to 30 percent chromium and from about 0 to 14 percent nickel.

Since many modifications are possible in the process and product of my invention as above described without departing from the scope of the invention, it is intended that the above description of my invention should be interpreted as illustrative, and the invention is not to be limited except as set forth in the claims which follow.

I claim:

1. A composition suitable for use in aqueous acid solution for the pickling of stainless steel, comprising a chlorate capable of furnishing chlorate ions in aqueous acid solution, and a fluoride salt capable of furnishing fluoride ions in aqueous acid solution, the chlorate and the fluoride being present in the composition in such proportion that from .02 to 40 parts by weight chlorate ion are furnished per part by weight of aqueous acid solution, the chlorate and the fluoride being present in the composition in such proportion that from .02 to 12 parts by weight chlorate ion are furnished per part by weight of fluoride ion in aqueous acid solution.

3. A pickling bath for removing oxide scale from stainless steel objects, consisting of an aqueous solution containing from about 0.01 to about 0.5 percent by weight hydrogen ion, from about 0.1 to about 6 percent by weight chlorate ion furnished by a chlorate soluble in said aqueous solution and from about 0.5 to about 6 percent by weight fluoride ion furnished by a fluoride salt soluble in said aqueous solution.

4. A solid composition suitable for production of an aqueous acid pickling solution for the pickling of stainless steel, comprising sodium chlorate and a sodium fluoride, the chlorate and the fluoride being present in the composition in the proportion that furnishes, in aqueous acid solution, from about to 6 parts by weight chlorate ion per part by weight fluoride ion.

5. A pickling bath for removing oxide scale from stainless steel objects, consisting of an aqueous solution containing from about 0.01 to 0.5% by weight of hydrogen ion, from about 0.1 to about 6% by weight of chlorate ion furnished by an alkali metal chlorate and from about 0.5 to about 6% by weight of fluoride ion furnished by a fluoride salt soluble in said aqueous solution.

FREDERICK C. ABBOTT.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,070,871 Stockton Feb. 16, 1937 2,395,694 Spence et a1 Feb. 26, 1946 2,408,424 Healy et a1. Oct. 1, 1946 

